1. Field of the Invention
The invention relates generally to systems and methods for the reduction of pollutants in the flue gas of combustion processes.
2. Discussion of Prior Art
A majority of utility boilers in the world burn coal or oil for their heat source. Combustion of coal or oil creates gaseous products of combustion. The major constituents of these gasses are nitrogen oxides and carbon dioxide. Also contained in the flue gas stream are various pollutants, the quantity of which is largely dependent on the constituents contained in the fuel burned. Many of these utility boilers are utilized as power plants to provide electricity.
Current regulations limit the quantity of nitrogen oxides, sulfur oxides, and particulate matter that can be emitted by a utility boiler. Additionally, some regions limit pollutants responsible for haze or the like. Regulations limiting the emission of mercury are either already in place in various states, and are pending at the federal level. Additional regulations are pending that relate to acid gasses and hazardous air pollutants, such as heavy metals in gaseous form.
Particulate matter can be conventionally removed using an electrostatic precipitator or in some cases a fabric filter. With respect to the gaseous pollutants, the conventional removal approach is to cool the products of combustion by evaporating water injected into a scrubber, such as a wet scrubber. Slurry of water that contains sorbents can be injected into the hot gas. Using this method, the gas is cooled by evaporation of water, from the temperature at the air heater outlet, to the saturation temperature of the gas. At the saturation temperature, many of the pollutants that were gasses condense into saturated droplets. At this point in the scrubbing process, many of the pollutants are removed from the flue gas stream as a component of the water circulating in the scrubber tower or as a sulfite or sulfate particle. However, scrubbers may not be effective at removing acid gasses from the combustion products.
Cooling of the flue gas stream through evaporation requires significant quantities of clean fresh water. The heat energy in the hot flue gas is wasted to atmosphere during the evaporation process. Additionally, localized high humidity levels can create fogging and other isolated weather phenomenon. As a result, the evaporation cooling method wastes energy and water.